1. Field of the Invention
The field of the invention is data processing, or, more specifically, methods, apparatus, and products for tuning a programmable power line filter.
2. Description of Related Art
A challenge for marketability of information technology equipment is the difficulty of meeting safety and electromagnetic compatibility regulatory requirements as mandated by many worldwide governing regulatory bodies. One such requirement, and of particular interest in a power supply is its electromagnetic interference (EMI) profile. For this reason, power supply designs often incorporate at their front-end an EMI filter, which is referred to in this specification as a power line filter.
A very common load for a power line filter is a switching power supply. A switching power supply is generally small in size, of light weight, and operates at high efficiency. In view of these advantages, a switching power supply is often preferred for electronic devices. It is well known, however, that harmful high order harmonic noises are induced from a switching transistor, a rectifying diode, a transformer, a choke coil or the like in normal operation of a switching power supply, potentially causing interference either with operation of a device powered by the switching power supply or interfering with the operations of other nearby devices. Such noises include both differential mode noise which flows between power lines (i.e., between a live line and a neutral line) and common mode noise which flows between a power line and a ground line. Such current noises, connected without filtering directly to power mains, represent additional high frequency current drains out of the mains. These noise currents cause a voltage drop at the source impedance of the mains which can be measured at the mains terminals.
To attenuate both the differential mode noise and the common mode noise, a power line filter is imposed between the power mains and a load. Over time, however, due to normal wear and tear and also due to changes in load conditions imposed by usage, load impedance values can change, reflecting impedance mismatches back through the filter, changing the operating characteristics of the filter and decreasing filter performance. Current power line filter design topologies are such that any post-design adjustment of the filter transfer function and/or resonance requires a physical modification of its circuit components.